Fast Fourier Transforms

FFT Page
Links and various Fortran FFT implementations.
http://faculty.prairiestate.edu/skifowit/fft/

fftpack
A package of Fortran subprograms for the fast Fourier transform of periodic and other symmetric sequences.
http://www.netlib.org/fftpack/

FFTW
Fast free C FFT library; includes multi-dimensional, real-complex, and parallel transforms.
http://www.fftw.org/

Jens J. Nielsen's Home Page
Arbitrary length FFT C-source, 3D gravity simulator, quantization of FIR filter coefficients. C source.
http://home.get2net.dk/jjn

Joerg's FXT Page
Source code and links for many different FFT implementations and related information.
http://www.jjj.de/fxt/

Row-oriented Multidimensional Fast Transform Library (RMFTL)
Power-of-two real and complex FFT software that is faster, and more accurate, than conventional iterative implementation of one-dimensional FFTs. Based on a proprietary algorithm.
http://www.davis-inc.com/rmftl

Sigview - FFT Analysis Software
A real-time spectral analysis application with spectral analysis tools, statistical functions and visualization system. It can analyse data from WAV files or sound input from microphone or CD player. Shareware.
http://www.sigview.com/